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Japan's nuclear disaster is not as bad as Chernobyl, but it's the worst since. The recent 8.9-magnitude earthquake and tsunami that followed have severely damaged the Fukushima Daiichi nuclear power plant. It has suffered two explosions, with warnings of a third possible, and fuel rods are exposed. Over 20,000 people have been evacuated from the area. This crisis raises important questions about the future of nuclear power and our failures not just to prepare for natural disasters but also possible failures in nuclear security. Harvard Kennedy School's Matthew Bunn gives us six key points to consider, originally published on the Power & Policy blog.

1. Not like Chernobyl

Bad as it is, this accident is dramatically less catastrophic than Chernobyl. That accident spread millions of curies of radioactivity — 3 to 4 percent of all the radioactivity in the reactor core — around the surrounding countryside, exposing millions of people in several countries. Large areas are uninhabitable to this day. Here there is no real prospect of a runaway chain reaction as occurred at Chernobyl. Instead, what has happened is melting of fuel in reactor cores, leading to release of a very modest amount of cesium and other fission products.

There is still a possibility of a larger release, if melted fuel falls to the bottom of the reactor and manages to burn through the containment, contacting water and creating radioactive steam. At present, it seems more likely than not (though the situation is still changing) that most of the evacuated people will be able to return to their homes and live their lives as before.

2. Worst nuclear disaster since Chernobyl

At the same time, this is the worst nuclear accident since Chernobyl, and in some respects worse than Three Mile Island (TMI). At TMI, they managed to avoid a hydrogen explosion; in this case, Japan had hydrogen explosions that destroyed much of the buildings at two reactors (though not the steel pressure vessels around the reactors themselves).

At TMI, there was only a very modest release of radiation. Here, you have at least one worker reportedly suffering from substantial radiation sickness and scores of people whose exposure is being carefully assessed and clear signs that there has been some significant release of radioactivity, including cesium and iodine (though reports seem to conflict as to how much). At TMI, one reactor's core was destroyed. Here, it appears at least two have suffered fuel melting, a third reportedly has exposed fuel, and others are facing serious loss of cooling.

3. "Defense in depth" is crucial — but sometimes not as deep as expected

This is clearly an example of the huge importance of taking redundant safety systems seriously, and considering carefully the full scope of events that could occur. Given the huge magnitude of the quake, I think it is impressive that all the affected reactors initially managed to shut down automatically as planned, and begin emergency cooling operations. None of the reactors, for example, suffered damage that prevented the insertion of the control rods. When the Fukushima-1 lost power, the backup diesel generators started up as planned. But they were then knocked out an hour later, apparently by the tsunami.

The reactors suffered, in effect, a one-two punch that hadn't really been expected. Clearly, given that an earthquake might well cause a tsunami, the diesel generators should have been designed in a way that would not be affected by tsunami waves. This is very likely a broader issue, that people have not adequately thought through the possibility of multiple traumas that could be caused by the same initiating event (e.g., a blackout and a large object crashing into the diesel generator as a result of a tornado — one could imagine many such coupled events).

This reinforces the view that whenever someone says there is less than a one-in-a-million chance of a complex system failing, there is more than a one-in-a-million chance they have made unjustified assumptions in their estimate.

4. We're less prepared for security incidents

The reason that the disaster hasn't been worse was that the Japanese system had many, many safety precautions in place. Japanese reactors are required to be designed and built to survive substantial earthquake accelerations. They are required to have backup power systems in place. And so on and so on. Everyone in the nuclear industry is trained to think about safety from day one.

Security, by contrast, is something most people in the nuclear industry might get a half-hour briefing on once a year. If you have intelligent adversaries, it wouldn't be a surprise to have the main power for the cooling and the backup power both fail — the adversaries would plan to make sure that happened. This transforms all the probabilities we rely on for safety. The global nuclear safety regime needs strengthening — but it is far stronger than the global nuclear security regime. And that applies to theft of nuclear material as well as sabotage of facilities.

On the other hand, only the power of Mother Nature would likely be able to cause the scale of damage we've seen at several reactors at once.

5. Psychological impacts probably most widespread

There may be several people who die or who are severely sickened by this accident — though that should of course be placed in the context of the more than 10,000 that are feared dead as a result of the quake and tsunami themselves. But the broader impact is likely to be psychological — from the stress and fear that tens or hundreds of thousands of people are suffering.

I would argue that the biggest health impacts of Chernobyl, despite the substantial radiation doses that some people received, were the stress, depression, and alcoholism that followed. The average person's great fear of radiation can have real health consequences.

6. Potentially severe impact on nuclear power's role in responding to climate change

It remains to be seen what impact this will have on the future of nuclear power in Japan, and the future of nuclear power elsewhere. China will likely continue its ambitious plans, for example. But if I had to guess, I would say public and investor perceptions of the safety of nuclear power around the world has been dealt a serious and lasting blow.

This did not take place in a developing country that had just built its first plant and hadn't had time to develop a proper safety culture. This took place in Japan, one of the wealthiest, most experienced, and most safety-conscious nations on earth (though one that also has had a history of safety issues that were covered up and not reported to the regulator in a timely way).

Admittedly these were old reactor designs and the new reactors people are considering building today would be safer — but whether that logic will address the public perception remains to be seen. My guess is that while we will still see some growth of nuclear power in some places, the prospects for growth on the scale required for nuclear to be even a noticeable part of the answer to mitigating greenhouse gas emissions have been substantially reduced.